Can a Simple Model Explain the Advent of Cells?

Scientists still don't know how a few biomolecules got together to kick-start life. Now a pair of theoretical biophysicists have come up with a simple mathematical model of two interacting chemicals that seems to replicate an essential part of the rise of life: the emergence of primitive, reproducing "protocells."

The first task in sparking life, or at least lifelike chemical interactions, would be to coax complex molecules to reproduce themselves from the other chemicals in solution around them. Under the right conditions, some proteins and other complex molecules can produce copies of themselves by purely chemical means. But making such a process run on as reliably as, say, bacteria multiplying in a petri dish is harder than one might guess.

That's because in the process, numerous copying errors, or mutations, arise. Occasionally, a mutation is an improvement that makes the molecule more likely to reproduce, and the mutation spreads throughout the population. Most, however, hinder a molecule's ability to survive or reproduce, and decades of experiments and theoretical models show that in a simple chemical solution, these bad mutations would accumulate over time and inevitably snuff out reproduction.

Of course, nature found a way around this problem, and over the years scientists have proposed several refinements to make their models more stable. For example, instead of one type of molecule reproducing, there could have been two molecules, each of them able to duplicate itself only if the other one was present. In that case, errors would not accumulate because when one molecule produced a defective copy of itself, that defective copy would not stimulate the second molecule to reproduce. The local concentration of the second molecule would then fall, limiting the defective first molecule's ability to reproduce itself.

But this model, too, has a problem. Many defective copies would still be produced. Although they would not replicate, they could crowd the reaction space, keeping the good copies from reaching each other. Since the 1970s, researchers have realized that they could get around this problem as well if the two chemicals formed globs, or "protocells." Then, protocells polluted with too many useless mutants would die out, while those with relatively few such mutants would continue to reproduce. But how do the molecules get it together to segregate themselves into protocells in the first place?

Theoretical biophysicists Atsushi Kamimura and Kunihiko Kaneko of the University of Tokyo think they have an answer. They had previously assumed that one of two hypothetical molecules would reproduce very slowly but would last much longer than the other did. In this way, the first molecule would act a bit like genetic information that is passed from generation to generation. Now, they argue that the model also naturally produces protocells.

That's because when the first, slowly reproducing molecule copies itself, the original molecule and the copy have time to drift apart before either reproduces again, preventing crowding. Around each of these slow-reproducing molecules, a cloud of the fast-reproducing second molecule emerges to create a spherical blob with the lone copy of the first molecule at its center. Empty space opens up between the blobs where there is none of the first molecule to support the reproduction of the second. To show that this scenario works, Kamimura and Kaneko ran computer simulations in which they varied the properties of the two molecules to find the conditions under which the protocells flourish or die out, as they reported online 29 December in Physical Review Letters.

"It's a beautiful example that shows that with relatively simple assumptions concerning chemical reactions, if you couple them in a proper way, you will actually be able to generate lifelike behavior." says Steen Rasmussen, a physicist and director of the Center for Fundamental Living Technology at the University of Southern Denmark, Odense. But, he cautions, "It's a thought experiment. If you want to go out and implement such a system in the lab, you can't do that from such an abstract model." In fact, in Kamimura and Kaneko's model, the two molecules are assumed to be nothing more than spheres that somehow catalyze each other's reproduction. That's a long way from DNA and RNA.